Set up a second/additional email account and send yourself a message. Does it send? Does it bounce? Under Mail pref's/Account Information tab, what is under Server Settings (button at bottom)? Mine has "Password" for authentication with Port 25; I suspect that's standard. But under "Advanced" tab, Port 110 uses MD-5 Challenge Response. I have no idea what that is, but it works for me.
Verify with Comcast the EXACT POP server names, and have them verify your settings. Since you can send, but not receive, the problem has to be with server settings/names/passwords, etc. Not an issue of being off-line.
Did it ever work properly, or did this suddenly come up in October?